Although the concept of variable-speed drives, based on utilization of multiphase machines, dates back to the late 1960s, it was not until the mid- to late 1990s that multiphase drives became serious contenders for various applications. These include electric ship propulsion, locomotive traction, electric and hybrid electric vehicles, ldquomore-electricrdquo aircraft, and high-power industrial applications. As a consequence, there has been a substantial increase in the interest for such drive systems worldwide, resulting in a huge volume of work published during the last ten years. An attempt is made in this paper to provide a brief review of the current state of the art in the area. After addressing the reasons for potential use of multiphase rather than three-phase drives and the available approaches to multiphase machine designs, various control schemes are surveyed. This is followed by a discussion of the multiphase voltage source inverter control. Various possibilities for the use of additional degrees of freedom that exist in multiphase machines are further elaborated. Finally, multiphase machine applications in electric energy generation are addressed.

Multiphase Electric Machines for Variable-Speed Applications

The area of multiphase variable-speed motor drives in general and multiphase induction motor drives in particular has experienced a substantial growth since the beginning of this century. Research has been conducted worldwide and numerous interesting developments have been reported in the literature. An attempt is made to provide a detailed overview of the current state-of-the-art in this area. The elaborated aspects include advantages of multiphase induction machines, modelling of multiphase induction machines, basic vector control and direct torque control schemes and PWM control of multiphase voltage source inverters. The authors also provide a detailed survey of the control strategies for five-phase and asymmetrical six-phase induction motor drives, as well as an overview of the approaches to the design of fault tolerant strategies for post-fault drive operation, and a discussion of multiphase multi-motor drives with single inverter supply. Experimental results, collected from various multiphase induction motor drive laboratory rigs, are also included to facilitate the understanding of the drive operation.

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Multiphase induction motor drives - : a technology status review

This paper addresses to some of the latest contributions on the application of Finite Control Set Model Predictive Control (FCS-MPC) in Power Electronics. In FCS-MPC , the switching states are directly applied to the power converter, without the need of an additional modulation stage. The paper shows how the use of FCS-MPC provides a simple and efficient computational realization for different control objectives in Power Electronics. Some applications of this technology in drives, active filters, power conditioning, distributed generation and renewable energy are covered. Finally, attention is paid to the discussion of new trends in this technology and to the identification of open questions and future research topics.

State of the Art of Finite Control Set Model Predictive Control in Power Electronics

This paper presents an overview of motor drive technologies used for safety-critical aerospace applications, with a particular focus placed on the choice of candidate machines and their drive topologies. Aircraft applications demand high reliability, high availability, and high power density while aiming to reduce weight, complexity, fuel consumption, operational costs, and environmental impact. New electric driven systems can meet these requirements and also provide significant technical and economic improvements over conventional mechanical, hydraulic, or pneumatic systems. Fault-tolerant motor drives can be achieved by partitioning and redundancy through the use of multichannel three-phase systems or multiple single-phase modules. Analytical methods are adopted to compare caged induction, reluctance, and PM motor technologies and their relative merits. The analysis suggests that the dual (or triple) three-phase PMAC motor drive may be a favored choice for general aerospace applications, striking a balance between necessary redundancy and undue complexity, while maintaining a balanced operation following a failure. The modular single-phase approach offers a good compromise between size and complexity but suffers from high total harmonic distortion of the supply and high torque ripple when faulted. For each specific aircraft application, a parametrical optimization of the suitable motor configuration is needed through a coupled electromagnetic and thermal analysis, and should be verified by finite-element analysis.

Overview of Electric Motor Technologies Used for More Electric Aircraft (MEA)

This paper presents a comprehensive study on the state-of-the-art and emerging wind energy technologies from the electrical engineering perspective. In an attempt to decrease cost of energy, increase the wind energy conversion efficiency, reliability, power density, and comply with the stringent grid codes, the electric generators and power electronic converters have emerged in a rigorous manner. From the market based survey, the most successful generator-converter configurations are addressed along with few promising topologies available in the literature. The back-to-back connected converters, passive generator-side converters, converters for multiphase generators, and converters without intermediate dc-link are investigated for high-power wind energy conversion systems (WECS), and presented in low and medium voltage category. The onshore and offshore wind farm configurations are analyzed with respect to the series/parallel connection of wind turbine ac/dc output terminals, and high voltage ac/dc transmission. The fault-ride through compliance methods used in the induction and synchronous generator based WECS are also discussed. The past, present and future trends in megawatt WECS are reviewed in terms of mechanical and electrical technologies, integration to power systems, and control theory. The important survey results, and technical merits and demerits of various WECS electrical systems are summarized by tables. The list of current and future wind turbines are also provided along with technical details.

High-Power Wind Energy Conversion Systems: State-of-the-Art and Emerging Technologies

A novel concept for multidrive systems, based on utilisation of multiphase machines, has been introduced recently. It has been shown that, by connecting in series stator windings of the multiphase machines in an appropriate manner, it becomes possible to control all the machines in the group independently using vector control principles, although the whole drive system is supplied from one multiphase voltage source inverter (VSI). The concept has been investigated so far only for true n-phase machines (i.e. machines with spatial displacement between any two consecutive phases equal to 2π/n) and all the available considerations are restricted to the inverter current control in the stationary reference frame. Moreover, all the available proofs of the decoupled dynamic control within these multidrive systems are simulation based. One specific case is discussed in detail; a two-motor series-connected six-phase drive supplied from a six-phase VSI. The two-motor drive system, based on utilisation of a true six-phase machine is considered first and, in addition to inverter current control in the stationary reference frame, inverter current control in the rotating reference frame is also analysed. It is shown that this method of current control requires modifications of the decoupling voltage terms, compared to those valid for a one-motor drive, this being caused by series connection of the two machines. Next, two-motor drives based on utilisation of quasi six-phase machines (with two three-phase stator windings displaced by 30°) are discussed and appropriate connection diagrams are developed. Verification of these schemes is provided by simulation. Finally, an experimental rig, which utilises a true six-phase machine connected in series with a three-phase machine and inverter current control in the stationary reference frame, is described and experimental verification of the decoupled dynamics of the two machines is provided by extensive testing.

Vector control schemes for series-connected six-phase two-motor drive systems

Summary Two-motor drive systems, which require independent control of the two machines, are nowadays traditionally realised by using two three-phase voltage source inverters supplying independently two machines, paralleled to the common DC-link (Fig. 1). However, application of power electronics in electric drives enables utilisation of AC-machines with a phase number higher than three. If the number of phases is increased to five, an entirely different solution for the realisation of a two-motor drive system becomes feasible. It is shown in the paper that an increase of the stator phase number to five enables completely independent vector control of two five-phase machines that are supplied from a single current-controlled voltage source inverter. In order to achieve such an independent control it is necessary to connect five-phase stator windings of the two machines in series and perform an appropriate phase sequence transposition (Fig. 2). The concept is equally applicable to any five-phase AC machine type and its major advantage, compared with an equivalent two-motor three-phase drive, is the saving of one inverter leg. Instead of six inverter legs, only five are required. Detailed verification of the novel five-phase two-motor drive configuration is provided by simulating the operation in the torque and speed mode, using indirect rotor flux oriented control principles. The concept can be extended to higher number of phases in a simple manner. Its main advantages and drawbacks are addressed as well.

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A Five-Phase Two-Machine Vector Controlled Induction Moto Drive Supplied from a Single Inverter

Two series-connected two-motor drive systems with a single inverter supply have been proposed recently. The first one consists of two five-phase machines supplied from a single five-phase inverter, whereas the second one comprises one symmetrical six-phase machine, a three-phase machine, and a single six-phase inverter. It has been verified experimentally that, by introducing an appropriate phase transposition in the series connection, completely independent dynamic control of the two machines can be realized using principles of vector control. Detailed dynamic models in the stationary common reference frame have also been reported for the two drive systems. These are taken here as the starting point, and equivalent circuit steady-state representation of the series-connected two-motor drive systems is developed. The equivalent circuits can then be used to evaluate steady-state characteristics of the drives in the same manner as it is done for three-phase induction machines, as well as for the inverter design. The dc-link voltage required for operation of the two series-connected machines is investigated next. It is shown that the specific winding connections lead to a dc voltage requirement that is smaller than it would have been had the windings of the two machines been connected directly in series. Experimental results, obtained during steady-state operation of the series-connected five-phase and six-phase two-motor drives, are presented. Developed equivalent circuit representation is finally verified by comparing calculated and experimentally obtained values for various voltages of the two-motor drives.

Steady State Modeling of Series-Connected Five-Phase and Six-Phase Two-Motor Drives

Control of an ac machine requires only two currents. It is, therefore, possible to achieve independent control of a number of three-phase machines using various inverter topologies with a reduced switch count. One such drive system comprises n three-phase induction machines supplied by a (2n + 1)-leg voltage source inverter (VSI). In such a configuration, one inverter leg is common for all machines. This paper presents a general pulsewidth modulation (PWM) method that is applicable for any number of machines n in the group. It is based on utilization of the standard three-phase modulators in conjunction with additional logic to generate modulation signals for all (2n + 1) legs of the inverter, and it enables an arbitrary distribution of the available dc bus voltage between the n machines. A developed PWM method has been implemented and tested in two-machine, three-machine, and four-machine drives, supplied from five-leg, seven-leg, and nine-leg VSIs, respectively. Experimental results are reported for open-loop V/f control, closed-loop current-controlled V/f mode of operation, and full vector control. It is shown that the nonideal nature of the VSI causes unsatisfactory operation of the drive system in the open-loop V/f mode, and that closed-loop current control is, therefore, preferred in this type of a drive system. It is believed that the four-machine drive has a good potential for industrial applications since, in certain cases, it enables a considerable saving in the required number of switches and installed inverter power.

A General PWM Method for a $(2n + 1)$ -Leg Inverter Supplying $n$ Three-Phase Machines

Independently controlled multimotor drives are typically realized by using a common dc link and independent sets of three-phase inverters and motors. In the case of an open-circuit fault in an inverter leg, one motor becomes single phase. To enable continued controllable operation by eliminating single phasing, the supply for the motor phase with the faulted inverter leg can be paralleled to a healthy leg of another inverter using hardware reconfiguration. Hence, the two motors are now supplied from a five-leg inverter, which has inherent voltage and current limitations. Theoretically, violating the voltage limit leads to inverter overmodulation and large torque oscillations. It is shown here that the finite-control-set model predictive control, designed to control the machines' stator flux and torque, can consider the inherent voltage limit dynamically in the control loop. Apart from preserving the independent control of the two machines, the additional constraint consideration significantly widens the operating speed ranges of the machines. In particular, it is shown that, whenever the voltage limit is entered, the controller reduces the stator flux level automatically, without requiring external flux reference change. The obtained performance is illustrated using experimental results and is also compared to the conventional two-motor field-oriented control scheme. The control concept is thus fully experimentally verified.

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Martin Jones

Papers

Vector control schemes for series-connected six-phase two-motor drive systems

A Five-Phase Two-Machine Vector Controlled Induction Moto Drive Supplied from a Single Inverter

Steady State Modeling of Series-Connected Five-Phase and Six-Phase Two-Motor Drives

A General PWM Method for a $(2n + 1)$ -Leg Inverter Supplying $n$ Three-Phase Machines

A fault-tolerant two-motor drive with FCS-MP-based flux and torque control